Question

how to find GCD of 406, 870

Answer 1

Factorize them first.
406=2*7*29. 
870=2*3*5*29 
Now take the biggest power of each prime factor which is common to both and multiply them.
Here we see they are $2 ^{1} and 29^{1}$.
So G.C.D=2*29=58.
Ans:58.

Answer 2

406= 2x7x29
870=2x3x5x29

GCD= 2 x 29
        = 58